1. Field of the Invention
This invention relates to an improvement in the treatment of Parkinson""s disease and related disorders. More specifically, the present invention introduces novel formulations of the combination carbidopa and levodopa, the current mainstay of therapy.
2. Background and Prior Art
Parkinson""s disease is associated with the depletion of dopamine from cells in the corpus striatum. Since dopamine does not cross the blood brain barrier and cannot therefore be used to treat Parkinson""s disease, its immediate precursor, levodopa, is used instead because it penetrates the brain where it is decarboxylated to dopamine. But levodopa is also decarboxylated to dopamine in peripheral tissues and consequently only a small portion of administered levodopa is transported unchanged to the brain. This reaction can be blocked by carbidopa which inhibits decarboxylation of peripheral levodopa but cannot itself cross the blood brain barrier and has no effect on the metabolism of levodopa in the brain.
The combination of carbidopa and levodopa is considered to be the most effective treatment for symptoms of Parkinson""s disease (The Medical Letter, 35:31-34, 1993). Nevertheless, certain limitations become apparent within two to five years of initiating combination therapy. As the disease progresses, the benefit from each dose becomes shorter (xe2x80x9cthe wearing off effectxe2x80x9d) and some patients fluctuate unpredictably between mobility and immobility (xe2x80x9cthe on-off effectxe2x80x9d). xe2x80x9cOnxe2x80x9d periods are usually associated with high plasma levodopa concentrations and often include abnormal involuntary movements, i.e., dyskinesias. xe2x80x9cOffxe2x80x9d periods have been correlated with low plasma levodopa and bradykinetic episodes.
In an effort to reduce the occurrence of xe2x80x9cwearing offxe2x80x9d and xe2x80x9con-offxe2x80x9d phenomena, a controlled release oral dosage combination was introduced with claims of slow and simultaneous release of carbidopa and levodopa from the formulation (U.S. Pat. No. 4,900,755 issued Feb. 13, 1990). Data from clinical trials cited in the patent indicate that effective antiparkinson effects were achieved with fewer daily doses of the controlled release form as compared with the conventional combination.
Nevertheless, there remains a significant flaw in the therapeutic application of controlled release carbidopa-levodopa; that is the considerable delay in onset of action. Mean time to peak concentration in healthy elderly subjects was found to be two hours for controlled release carbidopa-levodopa and only 0.5 hours for the conventional form (Physicians Desk Ref., 47th Ed., p. 976, 1993). A controlled release dosage form that could also provide rapid onset of action, at least equivalent to that of conventional carbidopa-levodopa would have an obvious clinical advantage over current therapy.
The strategy proposed in the present invention is it to formulate oral dosage forms containing both immediate release carbidopa-levodopa and controlled release carbidopa-levodopa. Ingestion would provide rapid onset antiparkinson activity via the immediate release component followed by sustained therapeutic activity from the controlled release component.
It is the purpose and principal object of this invention to provide an improved method for the treatment of Parkinson""s disease by using novel formulations of the combination carbidopa-levodopa which a) are effective in preventing the symptoms of Parkinson""s disease and yet which b) act rapidly avoiding significant onset delay common to the standard controlled release therapy.
The present invention concerns a method for treating Parkinsons""s disease using an oral dosage formulation comprising an immediate release layer of 10-25 mg of carbidopa and 50-200 mg of levodopa and a sustained release layer of 25-75 mg of carbidopa and 100-400 mg of levodopa whereby, following administration, carbidopa and levodopa are available for rapid and sustained therapeutic action.
An oral dosage formulation in the present method can be further characterized by a sustained release core depot of carbidopa-levodopa overcoated by an immediate release layer of carbidopa-levodopa.
Another aspect of the orally administering the dosage comprises administering a multilayer tablet comprising at least one layer of sustained release carbidopa-levodopa adjacent to at least one layer of immediate release carbidopa-levodopa. If desired, the layers in tablet are separated by an excipient layer.
Another aspect of the invention therefore concerns a pharmaceutical composition in oral dosage form for treating Parkinson""s disease, which comprises a combination of an immediate release portion of a combination of carbidopa and levodopa and a sustained release portion of a combination of carbidopa and levodopa and a pharmaceutically acceptable vehicle. The composition is effective in treating Parkinson""s disease. The dosage form is such that carbidopa and levodopa are available for immediate and sustained therapeutic action upon administration. The pharmaceutical composition can be in a dosage form that comprises a sustained release core portion of carbidopa and levodopa overcoated by an immediate release layer of carbidopa and levodopa. The pharmaceutical composition can also be in a dosage form that comprises a multilayer tablet which comprises at lease one layer of sustained release carbidopa-levodopa adjacent to at least one layer of immediate release carbidopa-levodopa.
In the pharmaceutical composition, in dosage form, an immediate release portion comprises about 10-25 mg of carbidopa and 50-200 mg of levodopa and a sustained release portion comprises about 25-75 mg of carbidopa and 100-400 mg of levodopa. The pharmaceutical composition in dosage form can comprise a sustained release core portion of carbidopa-levodopa overcoated by an immediate release layer of carbidopa-levodopa. The pharmaceutical composition in the dosage form can comprise a multilayer tablet of at least one layer of sustained release carbidopa-levodopa adjacent to at least one layer of immediate release carbidopa-levodopa.
The novel oral dosage formulations of the present invention each contain immediate release and controlled release components of the antiparkinson agents carbidopa (5-200 mg) and levodopa (25-600 mg). The conventional immediate release combination of carbidopa-levodopa reaches peak plasma concentrations in 30 minutes whereas the onset of the controlled release component is two hours followed by prolonged release over a four- to six-hour period.
The usual daily therapeutic dose of levodopa, when administered with carbidopa, is 300 to 750 mg and the dose of carbidopa approximately 75 mg per day but the latter is apparently devoid of adverse effects even at doses of 400 mg per day (J. E. Ahlskog, Hosp. Form., 27:146, 1992). Although the optimum daily dosage of carbidopa-levodopa must ultimately be determined by titrating each patient, a preferred range for twice daily maintenance therapy may include immediate release of 10-25 mg carbidopa and 50-200 mg levodopa and sustained release of 25-75 mg carbidopa and 100-400 mg levodopa.
Specific examples of these formulations are cited below. The amount and excipients listed can be changed through methods known to those skilled in the preparation of immediate and sustained release dosage forms. Some of these methods are available in Remington""s Pharmaceutical Sciences, 17th Ed., 1985, a standard reference in the field.